Permselective membranes for fluid separation are known and used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane for the upgrading of natural gas streams, separation of carbon dioxide from nitrogen from combustion exhaust gases, dehydration of alcohols, and the separation of hydrogen from various petrochemical and oil refining streams. For certain fluid streams, one or more components or minor contaminants may exhibit a strong interaction with the material of the membrane, which can plasticize the membrane. This can result in reduced productivity and selectivity and ultimately loss in membrane performance. Furthermore, some membrane materials may offer resistance to the interaction with contaminants but suffer from poor mechanical properties, thereby resulting in membrane failure when exposed to high membrane differential pressures and high temperatures. Other materials may not be capable of processing into membranes of the desired configuration (such as a hollow fiber membrane). Thus, a membrane having a good balance of high productivity and selectivity for the fluids of interest, persistently good separation performance despite long-term contact with aggressive process composition, pressure, and temperature conditions, and being made of a material having the ability to be processed into a wide variety of membrane configurations is highly desired.
Polymeric blending has traditionally been thought to be either problematic or of no benefit in the membrane field. This is primarily because different polymers are generally not miscible with one another, and for those few polymers that are miscible, a blend of the miscible polymers offers limited advantages for various reasons, including difficulty in blending, poor mechanical properties, or limited range of fluid transport properties.
The references discussed below describe separation membranes known in the art and disclose information relevant to production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane for the upgrading of natural gas streams, separation of carbon dioxide from nitrogen from combustion exhaust gases, dehydration of alcohols, and the separation of hydrogen from various petrochemical and oil refining streams. However, these references suffer from one or more of the disadvantages discussed above.
U.S. Pat. No. 4,705,540 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and a rigid dianhydride or mixtures thereof, specifically pyromellitic dianhydride (PMDA) and 4,4′-(hexafluoroisopropylidene)-bis (phthalic anhydride) (6FDA). These polyimides form membranes with high gas permeabilities but fairly low permselectivities. These polyimides are also sensitive to various organic solvents.
U.S. Pat. No. 4,880,442 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and essentially non-rigid dianhydrides. These polyimides again exhibit high gas permeabilities, but once again low permselectivities.
Bos et al., AlChE Journal, 47,1088 (2001), reports that polymer blends of two polyimides, Matrimid® 5218 polyimide (3,3′,4,4′-benzophenone tetracarboxylic dianhydride and diaminophenylindane) and copolyimide P84 [copolyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 80% toluenediisocyanate/20% 4,4′-methylene-bis(phenylisocyanate)], can increase the stability of the membrane against carbon dioxide plasticization when compared to the plain Matrimid® 5218 membrane.
U.S. Pat. No. 5,635,067 discloses a fluid separation membrane based on a blend of two distinct polyimides: P84 and Matrimid® 5218 polyimide. U.S. Pat. No. 7,018,445 also discloses fluid separation membrane based on a blend of polyimide: P84 with other polyimides. U.S. Pat. No. 7,393,383 discloses a fluid separation membrane based on a blend of P84 with polyamides or poly-imide-amides.
Barsema et al. (Journal of Membrane Science, 216 (2003), p 195-205) reports the permeation performance of dense film and asymmetric hollow fiber membranes made from P84.
Chung and Xu (Journal of Membrane Science 147, (1998) p 35-47) describe solution spinning of hollow fibers based on a miscible blend of PBI (2,2′-(m-phenylene)-5,5′-bibenzimadazole) and Ultem® 1010 polyetherimide. The performance of the spun fibers indicates non-integral separating layers so that even after post-treatment with silicone rubber (SiR) the membranes have unattractively low gas selectivity (O2/N2˜2 and H2/N2˜21).
Chung et al. (Journal of Membrane Science 271, (2006) p 221-231) show that incorporation of small amounts of PBI (3-17%) into Matrimid® 5218 polyimide increases temperature stability and selectivity of the blend film for pervaporation dehydration of t-butanol. PBI and Matrimid are reported to be miscible only at PBI compositions <18%. No gas separation data are reported for this system.
Wang et al. (Journal of Membrane Science 287 (2007) p 60-66) discusses solution spinning of a composite fiber comprising a distinct PBI-based outer sheath supported by a P84®-based (BTDA-TDI:MDI) core. The PBI sheath/P84 core composite fiber is claimed to have good performance for pervaporation dehydration of tetrafluoropropanol. No gas separation performance is reported. Fibers made from blends of PBI and P84 are not disclosed.
U.S. Pat. Nos. 6,946,015 and 6,997,971 claim increased permeance and selectivity for crosslinked PBI. The permselective PBI layer is supported on a microporous metal for high temperature gas separation. Such a membrane is relatively expensive to fabricate compared to an extruded polymeric hollow fiber.
Accordingly, it is highly desirable to create a membrane that can be used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane for the upgrading of natural gas streams, separation of carbon dioxide from nitrogen from combustion exhaust gases, dehydration of alcohols, and the separation of hydrogen from various petrochemical and oil refining streams. The desired membranes should exhibit a resistance to interaction of the material with the process and the resulting plasticizing of the membrane. Furthermore, membranes should have superior mechanical properties to allow the use of the membranes in high differential pressure applications, and be made of a material capable of being processed into the desired configuration (such as a hollow fiber membranes) Thus, membranes with a good balance of high productivity and selectivity for the fluids of interest, and persistently good separation performance despite long-term contact with aggressive process composition, pressure and temperature conditions are desired.